Thin film for forming transparent conductive films, such as indium tin oxide (ITO), ZnO-based, In2O3—ZnO-based, and MgO-based thin films, have been widely used as transparent electrodes of display devices mainly, for example, liquid crystal displays, touch panels, and EL displays. In many cases, oxide thin films, such as ITO thin films, for forming transparent conductive films are formed by sputtering.
In recent years, transparent conductive films for large-size flat panel displays (FPDs) are usually formed with in-line sputtering apparatuses. In the eighth and subsequent generations, from several to ten and several target plates each having a width of 150 to 300 mm and a length of 1500 mm or more are arranged, and a film is formed by moving a large-size glass substrate in the front of the targets.
It is difficult to produce such a large target as a single target plate. In general, several target pieces aligned in the longitudinal direction are bonded on a backing plate to form a single target-backing plate bonded body, and several bonded bodies are further aligned in the width direction into a large size.
A large target can be produced by arranging the target-backing plate bonded bodies in the width direction, however, the dividing lines in the several target pieces arranged in the longitudinal direction form lines with the corresponding dividing lines between target pieces of the adjacent target-backing plate bonded body. The dividing line involves a gap between the target pieces, and the target pieces differ from each other in the height though height difference is slight. Hence, a difference in level is generated with the dividing line as the boundary, and nodules are easily generated from the dividing line during sputtering. This causes adhesion of a large amount of dust, called particles, in a stripe shape on the film, resulting in a reduction in yield.
In general, from the easiness of production, target pieces having the same sizes are produced and are bonded onto a backing plate. In this case, the target pieces are finely aligned to give an apparently good appearance, however, all of dividing lines between adjoining target pieces are aligned and nodules are thereby concentrated at the area of aligned target pieces, which causes concentration of adhesion of dust called particles, at specific positions on the film, resulting in a reduction in yield.
The above-mentioned problem has been overlooked for a long time without becoming obvious, since the enhancement of productivity of target itself has been focused on and the production volume of large-size FPDs has been low in the past. In recent years, however, the defects in film formation have increased with an increase in production volume of large-size FPDs, and cases leading to a decrease in productivity have actually occurred.
In conventional technologies, Patent Literature 1 describes a large rectangular target plate that is a target assembly configured of a large number of tiles each meeting at interstices of three tiles or less, wherein the tiles are prevented from misalignment during thermal cycling. In this case, the tiles may be rectangular, square, hexagonal, or fan-shaped.
In Patent Literature 1, a large number of tiles are densely arranged for avoiding misalignment of the tiles, and no measures for preventing occurrence of nodules during sputtering and particles on a film are employed. In all FIGS. 6 to 10 showing examples of the target assemblies of Patent Literature 1, joining part of tiles are aligned at least between every other tile. Such a target assembly consequently causes occurrence of nodules during sputtering or increase of particles on a film.
Patent Literature 2 describes a disk-shaped divided-target composed of a circular fractional piece at the center and four fan-shaped fractional pieces at the periphery, which lack the central portion. It is proposed to join these fractional pieces by providing a difference in level at the joining part. Such a disk-shaped divided-target is unsuitable for producing a target assembly for large-size flat panel displays (FPDs).
In the divided-target of Patent Literature 2, the joining part of tiles are aligned between every other tile. The target having such a structure consequently has a problem of causing occurrence of nodules during sputtering or increase of particles on a film.
Patent Literature 3 describes a disk-shaped divided-target composed of a circular fractional piece at the center and two fan-shaped fractional pieces at the periphery, which lack the central portion. It is proposed to obliquely join these fractional pieces by providing a cut to each fractional piece at the joining part. However, such a disk-shaped divided-target as with the invention of Patent Literature 2 is unsuitable for producing a target assembly for large-size flat panel displays (FPDs).
In the divided-target of Patent Literature 3, the joining parts of tiles are aligned between every other tile. The target having such a structure consequently has a problem of causing occurrence of nodules during sputtering or increase of particles on a film.
These Patent Literatures are the same in that the targets are divided-targets each configured by arranging pieces on a single backing plate and do not arrive at the solution of the problem caused by an increase in size by arranging the target-backing plate bonded bodies in the width direction. Also it is problematic that no measures are employed to reduce the causes of occurrence of nodules during sputtering and increase of particles on a film.
Under such circumstances, the present inventors have filed a patent application for a sputtering target assembly (see Patent Literature 4), wherein a rectangular target plate having a width of 100 mm or more and a length of 1000 mm or more is composed of three or more target pieces A; a sputtering target-backing plate bonded body B is constituted by bonding the fractional target pieces A in the longitudinal direction on a backing plate such that the dividing lines lie in the width direction; and three or more of the sputtering target-backing plate bonded bodies B are further aligned in the width direction. The bonded bodies B are arranged into a sputtering target assembly in such a manner that the dividing lines between the three target pieces of one bonded body B are not present at the same positions of the dividing lines between fractional target pieces of adjacent another bonded body B.
This provides a significant effect of reducing defects due to occurrence of particles originated from target piece-bonding area. However, the structure of this sputtering target-backing plate bonded body needs a large width, which is a bit of a problem.